Colourblind no more — contrast and colour sensors for packaging and converting applications

In a black and white world, using photoelectric sensors would be very straightforward. Every photoelectric sensor would be specified with two sensing ranges, one for the black targets and one for the white targets. Since there would be no other colours, there would be no concern for the effect of colour on reflectivity and sensing range, or monitoring various shades of a colour, or identifying one colour out of a whole spectrum of possibilities.

In the real world, things are not so simple. In their technicolour lives, photoelectric sensors are often utilised to sort coloured products, identify coded markings and confirm the presence of an adhesive or date codes on a package. Unfortunately, they are not typically equipped to handle such demands. Vision systems offer a very reliable alternative, but with their four- and five-digit price tags they are often cost prohibitive.

Enter contrast sensors and colour sensors. Bridging the gap between photoelectric sensors and vision systems, they are equipped to detect even the slightest variations of the same colour. While a photoelectric sensor simply evaluates the amount of transmitted light that makes it to its receiver, a contrast or colour sensor must evaluate the type of reflected light. Contrast sensors detect differences between colours. Colour sensors detect a specific colour.

Contrast sensors, also referred to as colour mark or registration mark sensors, detect the difference between two colours, often corresponding to a target colour and a background colour. The threshold at which the sensor’s output changes state is typically halfway between the target colour’s and background colour’s reflected light values. Anything lighter than the switching threshold is one state, and anything darker is the opposite state. Colour sensors look for a characteristic response of one colour.

Contrast sensors often use RGB (red, green, blue) technology. This means that red, green, or blue light is emitted from the sensor’s transmitter and its receiver evaluates the light reflected off the target and received by the sensor. Contrast sensors operating on the RGB principle often automatically select the optimal colour light source for an application based on target and background colour.

Colour sensors are often based on the RGB principle as well. They typically send pulses from each of the three coloured LEDs and evaluate the responses before sending the next group of pulses. The reflected responses are compared to the programmed colour’s responses. More precise colour sensors require more samples of these pulse packets, which increases response time. Some high-precision colour sensors emit a white light to evaluate a more complete spectrum of the reflected light. Colour sensors are typically single-channel (single-colour) sensors, but more sophisticated models offer multiple channel programming for the identification of multiple colours.

Contrast sensors and colour sensors are often pigeonholed in traditional applications, while non-traditional but more powerful uses are overlooked. The most typical application for contrast sensors in packaging and converting is the detection of a registration mark on a web of media. The mark can be used to trigger a cutting, folding or gluing process for individual wrappers or cartons. Following are 10 less conventional but very powerful applications for contrast and colour sensors in the packaging and converting industries.

Adhesive verification A contrast sensor can easily verify the presence of adhesive. No glue or an inadequate amount of glue to hold a carton together can spell disaster for a packaging operation if containers fall apart. Contrast sensors evaluate the difference in reflectivity from the packaging media and the same media with a bead of glue on it (see Figure 1). Even a bead of clear epoxy just a few millimetres wide can be detected with a contrast sensor. A manufacturer may also have to detect a clear bead of glue on a clear plastic or mylar surface. Because of this ‘clear-on-clear’ scenario, a photoelectric sensor is ineffectual, but a luminescence sensor can easily detect many types of clear adhesives (see sidebar on luminescence sensors).

Figure 1: Adhesive verification using a contrast or luminescence sensor.

Date code/bar code check

Verifying that a date or lot code, bar code, expiration date or other printed information is present without the need to read it can be critical. A bar code reader or person reads and interprets the printed data, but using them to simply confirm the printed information is present is a costly endeavour. A contrast or colour sensor can be a simple and inexpensive way to detect the presence of markings on a package. The sensor would be used when the printed mark is always lighter or always darker than the package. The switching threshold is set between the mark colour and the package colour and will change state when the mark is present. When a package passes through the light spot of the sensor without any mark, its output will not turn on. A colour sensor can be utilised when the package colour changes but is not the same colour as the mark.

Tamper-proof seal confirmation Confirming the presence of a plastic wrapper placed over a pharmaceutical bottle cap is an invaluable safety check (see Figure 2). This seal ensures that the final product is safe and shows any evidence of tampering. Often this means a clear plastic film on a glossy container, with a difference in reflectivity too slight for even a contrast sensor. Luminescence sensors (see sidebar) can detect many of the films used in tamper-proof over-wraps. Because these wraps react to ultraviolet light emitted from the luminescence sensor, they react differently than the bottle.

Figure 2: Tamper-proof seal verification using a luminescence sensor.

Leaflet detection

Another important check in a packaging application is confirming a leaflet is inserted into a carton. For pharmaceutical products, the leaflets can contain safety information that is required by law, and missing leaflets means the product is not compliant with regulations. A luminescence sensor can detect a leaflet and ignore the package when the leaflet alone reacts to ultraviolet light. Some papers are inherently luminescent and, if they aren’t, they can be made luminescent by an invisible marking.

Print quality check

As caps on bottles are printed with a logo, the shade of the printed colour may become lighter or darker over time due to variances in the printing process. A contrast sensor with an analog output is a useful way to gauge the colour of a print for quality purposes. The light spot evaluates the reflectivity of each printed logo. The analog current output indicates how light or dark the print marks are on a scale, thus indicating when the print colour becomes too faded or too bright.

Lid insert verification

A lid or cap may have a plastic insert placed inside it to provide an optimal seal. A contrast sensor can be used to confirm the presence of the insert in the lid. When the insert is present, the contrast sensor sees one type of reflection, and it sees another when the insert is absent. A laser contrast sensor can provide an extended sensing range of several feet to differentiate the insert presence and absence (see Figure 3). This same sensor could also be used to detect protective packaging material (such as foam or plastic bubble webs).

Adhesive verification A contrast sensor can easily verify the presence of adhesive. No glue or an inadequate amount of glue to hold a carton together can spell disaster for a packaging operation if containers fall apart. Contrast sensors evaluate the difference in reflectivity from the packaging media and the same media with a bead of glue on it (see Figure 1). Even a bead of clear epoxy just a few millimetres wide can be detected with a contrast sensor. A manufacturer may also have to detect a clear bead of glue on a clear plastic or mylar surface. Because of this ‘clear-on-clear’ scenario, a photoelectric sensor is ineffectual, but a luminescence sensor can easily detect many types of clear adhesives (see sidebar on luminescence sensors).

Figure 3: Lid insert verification using a contrast sensor.

Shrink-wrap presence

To confirm that a pallet of boxes has been fully shrink-wrapped, a laser contrast sensor may be used to differentiate between the brown cardboard of the boxes and the glossy clear wrap over the boxes. The difference in reflectivity will indicate if the plastic wrap has been applied correctly.

Label presence and position

A pair of fibre-optic contrast sensors can confirm that a wraparound label is in place and properly aligned on a container. The two fibre-optic tips would be aimed at opposite borders of the label, so that if the label is askew, only one contrast sensor sees the label. If the label is missing, neither sensor detects the label. A trigger sensor, such as a standard retro-reflective photoelectric sensor, can be used to activate the contrast sensors when the container is in position.

Colour sorting

Variously coloured lids on paint canisters are sorted using a colour sensor to route them to the appropriate packing station. If more than two colour canisters are used, a colour sensor is the best choice to select one out of multiple other colours. While some colour sensors can be taught to recognise only one colour, other versions can be programmed for up to 10 separate colours to one sensor with an output for each colour.

Blister packaging

Colour sensors can be used to confirm the presence of pills or other products in clear blister packaging and to check their colours. If a pill is missing from a blister pocket, the colour sensor does not see the reflected colour response that was taught, and the package is flagged. A colour sensor could also confirm if chewing gum in blister packaging is green or white, for example.

Conclusion

Selecting a contrast, colour or luminescence sensor requires a clear understanding of the needs of a given application. Contrast sensors distinguish one colour against another background colour. Colour sensors detect the colour itself against a number of background colours and can be used to determine if the colour deviates. Luminescence sensors detect luminescent materials against non-luminescent backgrounds and are ideal for irregular or translucent surfaces. Contrast and colour sensors truly bridge the gap between photoelectric sensors and costly, complicated vision systems for a variety of packaging and converting uses.

Luminescence sensing Luminescence sensors make up a specific type of contrast sensors. Like contrast sensors, they distinguish two different conditions by emitting a light onto a target and evaluating the reflected light. However, luminescence sensors emit ultraviolet (UV) light, unlike the visible light spectrum emitted by colour and contrast sensors. Some materials, called luminophores, react specifically to UV light and are inherently present in the target or are added to it. These materials are stimulated by UV light and emit radiation in the visible light spectrum. This process is called photoluminescence. The colour or wavelength of the received light depends in part on the type of luminophore. So a luminescence sensor detects a target when the sensor emits UV light and receives visible light at a specific wavelength reflected back (see Figure 4). The targets used for luminescence sensors are often additives, but many materials are inherently luminescent, including some oils, epoxies, greases, inks, glues, chalks and detergents. Luminescence sensors detect markings that would be impossible using a standard photoelectric or contrast sensor. Marks on irregular backgrounds and invisible markings are easily sensed using a luminescence sensor. For example, detecting a mark on wood, normally very difficult due to irregular grains and colours, is straightforward with a luminescence sensor. Detecting a mark that is invisible, such as markings on aerosol container caps, is also easy. Figure 4: How a luminescence sensor works.

Pepperl+Fuchs (Aust) Pty Ltd
www.pepperl-fuchs.com